The present invention relates to a quick-extraction multiple punching head for converting punching machines from a single-punch to a multiple-punch configuration.
It is well-known that punching machines employed to machine metal plates use bodies, known as punch-holders, for accommodating the punches. The punches are located within the punch holder below the hammer, and, when struck by the hammer, the punches move over a stroke of a preset extent, allowing the metal plate to be worked.
Such punch-holders generally consist of a cylindrical body formed with a through seat accommodating the punch that is used for a given punching operation, so as to accommodate only one punch at a time. A horizontal disk-shaped member is mounted above the cylindrical body, such that it can move in a vertical direction, and supports on its lower face a mechanical coupling for engaging the head of the punch. Two helical pressure springs are located between the lower face of the disk and the upper face of the body of the punch-holder, and are arranged to effect a drawing upwards of the disk after each punching operation to extract the punch from the material being punched.
With this arrangement, when a given punching process has to be modified, it is necessary to remove the punch and replace it with another one suitable for performing the new desired punching process. Moreover, it should be noted that punches wear out over time and, in order to be used for longer periods, they are subjected to grinding or sharpening operations which cause variations in their overall useful length. All of this results in variations in the stroke that the hammer must perform to execute a complete punching operation. A variation of a few millimeters can be compensated by adjusting the stroke of the hammer.
The above-described prior art suffers from some drawbacks. A first drawback is the fact that replacement of a punch is relatively complicated and time-consuming.
A second drawback is that the extraction force exerted by the return springs is rather limited. Thus, it may happen that after an active descent of the punch, the punch is held jammed in a just perforated metal plate, consequently requiring that an operator manually release the punch from the metal plate by using tools, resulting in a significant waste of time and the necessity of stopping the punching machine.
A third drawback is that, after a limited number of grinding operations aimed at restoring their lower surfaces, the punches must be replaced because their shortening can no longer be compensated for by adjusting the stroke of the hammer.
GB 2,324,755 A discloses a device for converting punching machines from manual to quick and automatic, comprising a stator element inserted in a seat provided in the machine and supporting a plurality of punches provided peripherally, and a rotor element coaxial with the stator element provided with pusher means, for selection of the punch by gradual rotations thereof. The rotor element, suitable for being moved vertically by a pre-set extent, is guided by stem-like means mounted coaxially inside the stator. Punches are withdrawn by the elongation of a helical spring fitted onto the stem-like means. The force exerted by the spring is constant, regardless of the composition of the plate to be punched and of the plate's resistance thereof to punching.
U.S. Pat. No. 4,375,774 to Wilson discloses a single punch assembly comprising a punch guide, a punch head assembly, and a punch driver. The punch head assembly and the punch driver are threadably engaged with each other and allow adjustment of the punch length. The diameter of the punch head is much greater than that of the shank, and this makes the assembly cumbersome to use. Another drawback of this invention is the fact that the adjustment means are only suitable for application to single punching machines, as the threading engages components whose diameter is much greater than that of the punch itself.